Glyphosate herbicides include N-phosphonomethylglycine, sometimes referred to as “glyphosate acid”, salts and esters thereof, and other compounds which when applied to plants release or otherwise provide glyphosate ions. Long known as useful herbicides, glyphosate herbicides are generally applied in aqueous solution or dispersion, most commonly by spraying, to foliage of plants to be killed or controlled. Plants growing where they are not desired are herein referred to for convenience as “weeds”, but it should be recognized that glyphosate herbicides are frequently used for killing or controlling plants not normally thought of as weeds, such as volunteer plants of a crop species growing in a crop of another species, or pasture plants that have outlived their period of maximum productivity.
In most commercial glyphosate herbicide formulations, the glyphosate is present as a water-soluble salt. Herbicidal salts of glyphosate are disclosed, for example, in U.S. Pat. No. 3,799,758 to Franz, U.S. Pat. No. 3,853,530 to Franz, U.S. Pat. No. 4,140,513 to Prill, U.S. Pat. No. 4,315,765 to Large, U.S. Pat. No. 4,405,531 to Franz, U.S. Pat. No. 4,481,026 to Prisbylla and U.S. Pat. No. 4,507,250 to Bakel. Glyphosate salts disclosed include alkali metal, for example sodium and potassium, salts; ammonium salt; and numerous salts having an ammonium, sulfonium or sulfoxonium cation substituted with 1—3 organic groups containing in total 1-6 carbon atoms, for example dimethylammonium, isopropylammonium, ethanolammonium and trimethylsulfonium salts.
Commercial formulations of glyphosate salts include, for example, Roundup®, Accord®, Roundup® Ultra and Roundup® Xtra herbicides of Monsanto Company, which contain the isopropylammonium salt, Roundup® Dry and Rival® herbicides of Monsanto Company, which contain the ammonium salt, Roundup® Geoforce herbicide of Monsanto Company, which contains the sodium salt, and Touchdown® herbicide of Zeneca, which contains the trimethylsulfonium salt.
Glyphosate herbicides are in most cases formulated by the manufacturer, and/or mixed by the end user, with one or more surfactants. Surfactants assist in retention of droplets of the applied spray on the foliage of treated plants, adhesion of the spray droplets to the foliar surface and penetration of the water-soluble pesticide through the hydrophobic cuticle that covers the foliar surface, by these means and possibly in other ways enhancing herbicidal activity of the glyphosate. An extensive study by Wyrill & Burnside, Weed Science 25, 275-287, 1977 led to a conclusion that “an effective surfactant is a critical component of any glyphosate spray mixture”, but noted great variation among surfactant types in the degree of enhancement of herbicidal activity afforded. In general, cationic surfactants gave the greatest degree of enhancement. The authors also remarked that the effectiveness of combinations of surfactants was generally unpredictable, and warned against the “indiscriminate mixing of surfactants” in glyphosate spray compositions.
Among cationic surfactants known to be effective in enhancing activity of glyphosate herbicides are tertiary alkylamines, polyoxyethylene tertiary alkylamines, quaternary alkylammonium salts and polyoxyethylene quaternary alkylammonium salts. The term “alkyl” in the present context is used conventionally to refer to straight and branched chain, saturated and unsaturated aliphatic hydrocarbyl groups having about 8 to about 24 carbon atoms. Also effective are etheramine surfactants, in which the carbon chain of the hydrocarbyl group is interrupted by one or more ether linkages. Many other variants of such amine-based surfactants are also known, including polyoxypropylene quaternary ammonium salts, diamines and amphoteric types such as alkylamine oxides, alkylbetaines, etc., all having in common, like the cationic surfactants mentioned above, the possession of a cationic or protonatable amino group. Glyphosate herbicide compositions comprising one or more amine-based surfactants are disclosed illustratively in U.S. Pat. No. 5,118,444 to Nguyen, U.S. Pat. No. 5,317,003 to Kassebaum & Berk, U.S. Pat. No. 5,668,085 to Forbes et al., U.S. Pat. No. 5,750,468 to Wright et al., U.S. Pat. No. 5,821,195 to Sandbrink et al., Australian Patent No. 595406, and International Publication Nos. WO 95/33379, WO 97/05779, WO 97/31890, WO 97/36494, WO 98/24313, WO 98/53680, WO 99/00012 and WO 99/05914.
Anionic surfactants are in general weak potentiators of glyphosate herbicidal activity. However, International Publication No. WO 99/21423 discloses compositions comprising a glyphosate herbicide and an anionic N-acyl sarcosine or sarcosinate surfactant which are said to “maintain [the] herbicidal efficacy” of a commercial glyphosate herbicidal formulation. An example of such a composition, applied at 100 g glyphosate acid equivalent (a.e.) per acre (approximately 240 g a.e./ha), is reported therein to give 65% “overall kill” of mixed broadleaf weeds and grasses 21 days after application, by comparison with 62% “overall kill” achieved with the commercial glyphosate formulation Roundup® applied at the same rate. The weight ratio of N-alkyl sarcosinate to glyphosate a.e. in the disclosed composition is calculated to be approximately 1:16. At this very low surfactant to glyphosate ratio the composition had low eye irritancy potential by comparison with Roundup® herbicide which has a much higher surfactant concentration. Unfortunately no data are provided on the degree of “overall kill” provided in the absence of surfactant; without such data it is not possible to determine whether or not the N-alkyl sarcosinate was effective in enhancing herbicidal activity of glyphosate under the conditions of the test reported.
Combinations of cationic and certain anionic surfactants have previously been disclosed to provide, in combination with a glyphosate herbicide, formulations of low eye irritancy. For example, U.S. Pat. No. 5,683,958 to Berger & Jimenez discloses compositions comprising a glyphosate herbicide, a polyoxyethylene alkylamine surfactant and an alkyl sulfate, polyoxyethylene alkyl or alkylphenol sulfate, alkyl phosphate, polyoxyethylene alkyl or alkylphenol phosphate or polyoxyethylene alkyl or alkylphenol carboxylate surfactant; and U.S. Pat. No. 5,389,598 to Berk & Kassebaum discloses compositions comprising a glyphosate herbicide, a polyoxyethylene alkylamine surfactant and an alkyl mono- or dicarboxylic acid, for example a fatty acid, as having low eye irritancy. However, such compositions have not been disclosed to have herbicidal efficacy that is unexpectedly enhanced by comparison with otherwise similar compositions having only one of the two surfactant components.